1. Field of the Invention
The present invention relates to a digital-to-analog FIR filter. More specifically, the present invention relates to a digital-to-analog filter which uses active current division steering techniques.
2. Brief Description of the Related Technology
Previous methods for digital-to-analog (D/A) signal conversion and reconstruction filtering involve several methods. The actual D/A conversion process has been accomplished using methods such as a single current source and sink, or by dumping charge from a switched capacitor that has been charged to either a positive or negative reference voltage. Reconstruction filtering has been accomplished by using a combination of active and/or passive classical filtering techniques, such as continuous time active filters with resistors and capacitors, continuous time passive filters utilizing resistors, capacitors, and inductors, or switched capacitor filter techniques. A method of filtering which combines a D/A converter with a reconstruction filter has recently become known in the art.
Oversampled D/A converters generally include the following signal processing blocks: (1) an interpolator filter, or series of filters, which raises the sample rate of the incoming digital signal to a higher sample rate, (2) a digital sigma-delta processor (or noise shaper) which lowers the number of bits representing the signal by shaping the quantization noise in a way that places most of it at higher frequencies, (3) a D/A converter which converts the output of the noise shaper into an analog signal, and (4) an analog low pass filter which removes, or substantially lowers, the noise that was placed at higher frequencies by the noise shaper.
In all sigma-delta D/A converters, there exists a need to filter the high frequency noise inherent to this method of conversion. It is common for a digital noise shaper (digital sigma-delta modulator) to have as its output a single bit. The single bit digital output signal is then converted to an analog signal using switched capacitor techniques or switched current source techniques. Once this conversion is made, filtering of the high frequency noise is then accomplished through a variety of means.
As illustrated in FIG. 1, a semi-digital reconstruction filter typically uses a tapped delay line, or shift register, to control a plurality of devices each of which has an associated gain factor. The outputs of the plurality of devices are then summed together to form a single output of the filter. In some cases, individual current sources are employed as the plurality of devices. The amount of current in each current source is designed such that a desired FIR filter response is achieved. The output of each current source is then provided, or steered, to a current summing node (IOUT) or to an alternative current summing node (IOUT*), depending on the logic state of the control bit (B.sub.N) at the delay line tap associated with each current source. The currents at one or both of the current summing nodes are then converted to a voltage using standard current to voltage conversion techniques. Additional filtering may then be employed to remove extremely high frequency noise.
In other cases, the plurality of devices in the semi-digital filter FIR coefficients are represented as charge stored on a plurality of capacitors. The charge on each capacitor can then be summed by employing a switched capacitor summing amplifier. Once again, additional filtering may be employed to remove any extremely high frequency noise.
In another semi-digital filter scheme, the FIR coefficients are represented as a current value through a plurality of resistors. Each resistor is selectively connected to a voltage reference depending on the state of the individual control bit from the delay line tap associated with each individual resistor. The current is then summed and converted to a voltage by means of resistive feedback around an operational amplifier (op amp). As in the previous methods discussed, additional filtering may be employed to remove any high frequency noise.
Previous current steering semi-digital FIR filters utilize current paths having linear resistors, or resistive elements such as FETs or CMOS transmission gates biased in the linear region. This results in relatively low output impedance of the resistive elements of the filter. Any offset on inputs to an operational amplifier (op amp) connected to the current paths in the prior filter circuits may cause an error term. Since the current through each path is dependent on the resistance of the resistive element in each path, the state of the switches and the op amp offset, the FIR coefficients for the filter, as determined by the current in each path, are a function of the op amp offset. A need exists to minimize, or eliminate, this offset distortion term.